1. Field of Invention
The present invention relates to the field of rechargeable secondary batteries, particularly to secondary batteries containing a protective circuit.
2. Description of Related Art
Secondary batteries having a protective circuit are used in cellphones or portable personal computers and there are demands for safer protective circuits with higher charging capacity.
In FIG. 6, a secondary battery 101 of the prior art is depicted. The secondary battery 101 comprises an accumulator 107 and a fuse 111.
One end of fuse 111 is connected to a terminal of accumulator 107 on the high voltage side and the other end is connected to an external connection terminal 117 on the high voltage side. The terminal of accumulator 107 on the low voltage side is connected to an external connection terminal 118 on the low voltage side.
When an external DC voltage source 130 is connected to external connection terminals 117, 118, a current supplied from external DC voltage source 130 passes through fuse 111 to charge accumulator 107.
If external connection terminals 117, 118 are short-circuited or an external DC voltage source 130 outputs a voltage exceeding a prespecified voltage is connected to such a secondary battery 101, a large current passes through fuse 111, which causes fuse 111 to blow. This protects the accumulator 107, which otherwise would generally arise a safety problem when the accumulator in the secondary battery breaks.
However, fuse 111 must be changed if external connection terminals 117, 118 are short-circuited by accident in the production line or the like because secondary battery 101 becomes unusable when fuse 111 blows.
Secondary battery 102 shown in FIG. 7 comprises a thermistor 114 in place of fuse 111, and if external connection terminals 117, 118 are short-circuited or a high voltage is applied between external connection terminals 117, 118, the resistance value of thermistor 114 per se increases by a rise in the temperature of thermistor 114 to limit the charging/discharging current to accumulator 107.
When abnormal conditions such as short circuit or high voltage application are eliminated and thermistor 114 is restored to normal temperature, the resistance value of thermistor 114 returns to the original value to allow secondary battery 102 to be reused.
However, if an especially high voltage is applied between external connection terminals 117, 118, the high voltage is also applied across thermistor 114, which involves the danger of the breakage of thermistor 114 resulting in the breakage of accumulator 107.
The present invention overcomes the disadvantages of the prior art described above with the purpose of providing a highly safe secondary battery.